3phase and neutral

I accept your correction, 'bernard' to your former post, where you implied that the supply neutral is sometimes connected to the star link of a motor terminal block. You could have expressed it better though, thereby avoiding the need or such correction.


There could be any number of reasons for a starter to have a neutral connection. BAS has mentioned at least one.

Someone else has mentioned thermistor protection as a reason for the need for a neutral, but even this is 'non-starter', because imbedded thermistors have to be connected via their own control circuit which doesn't necessarily require a phase & neutral - and which wouldn't generally be found within a bog-standard DOL starter enclosure.


Lucia.
 
Sponsored Links
A three phase motor does not need a neutral because it has two 'phase' leads running it at any given point in time. The electricity comes in on one phase lead and goes out on the other phase lead.

Cobblers...
it has 3 phase leads running it at any given point, that's why it's called 3 phase..... :rolleyes:

the "neutral" for any given phase is the other 2 phases..
 
A three phase motor does not need a neutral because it has two 'phase' leads running it at any given point in time. The electricity comes in on one phase lead and goes out on the other phase lead.
3phase1.jpg
 
Sponsored Links
I have read with interest the debate on neutrals with three phase. To date on a motor not seen a neutral connected. However on an alternator it is quite common. It seems it reduces harmonics by rectifying the star point as well as the three phases and as a result reduces the heat generated within the alternator. A 45A alternator can be up-rated to 55A just by adding the extra diodes.

I can well see there being a similar harmonics problem with motors and as a result even though I have not seen a neutral connection I would not rule out there ever being connected.

I have seen many changes in my time. Not many years ago to run three phase machines on a single phase supply although not impossible was not an easy process with a large motor with flywheel being started first with the help of capacitors and centrifugal switches not really practicable. Yet today with the costs of inverters dropping the running of three phase motors from a single phase supply is now common.

We also add more and more into the motor casing and I have fitted motors with ASii control, Brakes, gearboxes, and the inverter all in built having a three phase and 24VDC supply with the comms superimposed onto the DC supply it also had inputs for limit switches and could be reversed remote by the PLC.

I have been caught out in the past. I was told we had some 8V batteries in to stores and told them no way either 6V or 12V for vehicles. I lost my £20 seemed Caterpillar made them to get brighter lights on 6V plant. Even had instructions on how to adjust the voltage regulator. Back in days of donkey engine starters on the 583 side booms.

I seem to remember SWF motors did use the neutral on the brake on some motors? Seem to remember some very odd test voltages like 183V where a part rectified supply was used for brake.

Hence although highly unlikely that a three phase motor has a neutral connected I would not stick out my neck and say never. It just might!
 

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.


Are you a trade or supplier? You can create your listing free at DIYnot Local

 
Back
Top